1. Field of the Invention
The present invention relates to improved catalysts and to improved process for preparing salts of aldonic acid. In one aspect of the invention, the invention relates to a process for making catalysts used in the preparation of those salts which catalyst results in high selectivity, high rate of oxidation and low colored product when aldose is oxidized to form the salt of aldonic acid. In a second aspect of the invention, the invention relates to adding small amounts of aldehyde during the oxidation of the aldose to the salt in order to improve the rate of oxidation of the catalyst and prevent its deactivation (i.e., increasing catalyst stability).
2. Background Discussion
Salts of aldonic acids (e.g., sodium lactobionate) are the precursor in the preparation of aldonolactone which are, in turn, starting materials used in the preparation of N-alkyl aldonamide surfactants (e.g., lactobionamides). Aldobionamides such as lactobionamides are nonionic surfactants which can be used in a variety of personal products and/or detergent products such as described, for example, in U.S. Ser. No. 981,737 to Au et al., hereby incorporated by reference into the subject application.
Supported noble metal catalysts such as Pd/C are not currently used commercially for the oxidation of lactose with oxygen due to poor performance of the catalyst (activity, selectivity and stability).
Salts of aldonic acid have been prepared from aldoses using palladium/bismuth/carbon catalysts.
EP 232,202, for example, describes the preparation of Pd/Bi/C catalyst (Pd:Bi of 6:2) made by impregnation of an aqueous slurry of Pd/C with bismuth subnitrate, dissolved in concentrated hydrochloric acid, followed by treatment of aqueous NaOH and reduction with formaldehyde. This catalyst was reported to give 97.5% conversion in the oxidation of maltose to sodium maltobionate. By contrast, using a catalyst prepared by the method of the invention, applicants have achieved a 99.7% conversion of lactose to sodium lactobionate having less color impurities. In addition, the present method eliminated the use of concentrated acid and base and costly bismuth subnitrate in the preparation of the catalyst.
European Patent EP 142,725 described the preparation of Bi/Pd/C catalyst made by impregnation of active carbon with bismuth hydroxide nitrate in concentrated HCl and Palladium chloride followed by neutralization with NaOH and reduction with formaldehyde. Both Pd/Bi/C and Bi/Pd/C catalysts described here show higher activity and selectivity in comparison with the unprompted Pd/C catalyst.
Henrickus E. J. "Carbohydrate Research", 204: 121-129 (1990) described the in situ bismuth promotion of Palladium on carbon catalyst. The author claimed this in situ promoted Pd/C gave 100% selectivity with 95% conversion in the oxidation of lactose to Na lactobionate.
However, all the Pd/Bi/C catalysts described above either show a lack of reactivity, selectivity or stability. Further, each involved using concentrate acids and bases in the preparation of the catalyst.
Accordingly, there is a need in the art to provide catalysts yielding even higher degree of selectivity (i.e., yield of product) in the conversion of aldose to salts of aldonic acid.
Further, there is a need in the art to provide catalyst yielding better rates of oxidation.
Finally, there is a need in the art for a method for preventing deactivation of the catalyst.